Electrophotographic rinse and method



United States Patent 3,357,828 ELECTROPHOTOGRAPHIC RINSE AND METHODClyde A. Moe, Sanford, Mich., assignor to The Dow Chemical Company,Midland, Mich., a corporation of Delaware No Drawing. Filed Nov. 9,1964, Ser. No. 409,963 7 Claims. (Cl. 96-1) This invention relates toelectrophotographic printing processes and more particularly isconcerned with a new and novel rinsing liquid having a high flash point,a high electrical resistivity, and a low toxicity, which is economicaland safe, thus highly useful for rinsing imaged electrophotographicplates.

Exemplary of an electrophotographic printing process for preparingphotoengraving plates is one wherein a photoconductive compositioncomprising, for example, a photoconductive zinc oxide suspended as afine dispersion of micron sized particles in a cross-linkable, filmforming, insulating silicone resin as a binder, is coated on, forinstance, a thin magnesium plate. The plate so-coated, commonly referredto as a recording element, is then negatively electrostatically chargedand exposed to an illuminated image desired to be reproduced, thereby toform a latent electrostatic image. The latent image is then developed,for example, by first contacting said latent image with triboelectriccatalyst particles of, for example, aluminum octanoate, suspended, forinstance, in a carrier liquid of n-heptane. Following said contact, thecoating, now catalyzed with said triboelectric catalyst in a patternconforming to the electrostatic image, is conventionally rinsed in, forexample, a liquid comprising primarily isooctane containing variousresinous additives in order to wash off the developing solution and theexcess deposition of catalyst. Thereafter, the rinsed plate is cured(cross-linked) by heating to convert the resin binder in the compositionto an acid etchant resist, followed by removal of the composition fromthe non-image areas of the coating, whereupon, the plate may be etched,for example, by means of the recently developed powderless etchingprocess.

The rinses heretofore used as hereinbefore described are substantiallyunacceptable in that several serious disadvantages attend their use. Forexample, these rinses are uneconomical requiring in some cases expensiveresinous additives. Moreover, they are highly flammable, thus rep resenta significant safety hazard. For example, isooctane has a flash point ofabout 12 F. In addition, some of the rinse ingredients are quite toxicto handle and inhale, thus, in addition to being flammable, aredeleterious to health as well. Also said rinses, in addition toproviding only a nominally acceptable rinsing action in general, do notprovide clean non-image areas and sharp images as are desired.

The term rinsing action as used herein refers to the overall rinsingeffectiveness of a given rinse with respect to (1) removing catalystfrom the non-image areas so that the resin in said areas will notcross-link during curing, and (2) removing only the catalyst material inthe image areas not electrically and/or poorly adhering thereto.

Ideally then, a rinsing liquid for the purpose heretofore set forthshould have a high flash point above, for example, about 100 F. Itshould thoroughly remove all (but only) excess catalyst, foreign matterand the like from both the image and non-image areas and have anevaporation rate such as to be capable of being air dried oil? the platewithin, for example, about 5-6 minutes or less after rinsing. Thoughsuch a rinse should have a high evaporation rate for fast drying, itshould not have an objectionable and irritating odor nor be toxic andirritating to the skin. Also, and very important, such a rinse shouldleach out of the coating as little of the resinous binder as possible,have a high electrical resistivity, and a suflicient rinse capacitysuch' that, for instance, 30 to 40 or more can be obtained from onerinse charge.

A principal object of the present invention, therefore, in the processfor preparing photoengraving plates, is to provide a novel and improved,high resistivity, high flash, and non-toxic rinse composition which ingeneral provides good rinsing action, and which, in addition, is alsoeconomical, eflicient, and safe.

This and other objects and advantages have been obtained by means of thenovel rinse composition of the present invention which comprises: (a)from about 47.5 to about 49.75 parts by volume of a fluorinatedhydrocarbon liquid, (b) from about 47.5 to about 49.75 parts by volumeof an isoparafiinic hydrocarbon liquid, and (c) from about 0.5 to about5.0 parts by volume of odorless mineral spirits.

Preferably, from about 49.25 to about 49.75 parts by volume of both thefluorinated hydrocarbon and the isoparafiinic liquids will be employedin formulating the present rinse together with about 0.5 to about 1.5parts by volume of odorless mineral spirits.

The fluorinated hydrocarbon liquid which is preferred for use in thepresent rinse is a stable fluorinated hydrocarbon corresponding to theformula CCI F-CCIF called Freon TF (trademark) which is a non-flammableliquid having a molecular weight of 187.29, a boiling point of 117.63 F.(at one atmosphere), an evaporation rate of 170 seconds (based on CClbeing a Kauri- Butanol value of 31, and which has a very low toxicitylevel.

Isopa-r E (trademark) is an isoparaflinic hydrocarbon which is preferredfor use in the invention. This is a synthetic hydrocarbon liquid of veryhigh purity, having an extremely low odor level, manufactured by theHumble Oil and Refining Co. The liquid has a boiling range of 240-290F., a flash point (Tag Closed Cup) of about 50 F., a Kauri-Butanol valueof about 29, and an evaporation rate at 100 percent of about 353seconds. In addition, it has a very low toxicity level.

The odorless mineral spirits which may be used herein is an aliphatichydrocarbon distributed by Western Solvents and Chemicals Co., having aboiling range of from about 349 to about 406 F., a flash point of about133 F. (TCC), a Kauri-Butanol value of about 25, and an evaporation rateat 100 percent of about 2400 seconds.

When employing the present invention, the rinse after being formulatedis thoroughly mixed, whereupon, it can be used immediately or be storedindefinitely before use. When used it is charged intoa suitable rinsetank, such as, for example, the type manufactured by the Master EtchingMachine Co. adapted to provide a uniform liquid curtain of the rinsecomposition through which the plate must pass upon being immersed in thetank so-charged and withdrawn. A plate to be rinsed, after having beencontacted with the aforesaid catalyst material, is then immersed in andwithdrawn from said tank, for example, from about 2 to about 10 times,and preferably 3 to 4 times, to provide inversely a suitable contacttime of, for example, from about 3 to about 8 seconds perimmersionwrthdrawal. Upon the last withdrawal the plate is allowed todrain and then dried either in air or by forced air, preferably thelatter. The plate so-rinsed is then subjected to the remaining steps ofpreparing a photoengraving plate as described above. The present novelrinse composition is good, for example, for rinsing a minimum of about30- 40 full size (18" x 24") plates, or flats as they are called, and issafe for use since it has a flash point of about 109 F.

The present invention, therefore, provides a high flash, highresistivity, non-toxic rinsing liquid highly useful in insing imaged anddeveloped photoconductive-coated lectrophotographic printing plates,containing in said oating a cross-linking binder and a photoconductivezinc xide. It provides, moreover, a novel and improved rinsing .quidhaving good rinsing action, which is economical, fficient, and safe suchthat when used in the electrophotolraphic preparation of, for example,photoengraving lates, a significant improvement is obtained.

The following examples further illustrate the present nvention but arenot to be construed as limiting the invenion thereto.

Example I A number of photoengraving grade magnesium plates :ach 18" x24" in size and 0.064 in thickness were spray :oated with a zincoxide-silicone binder photoconductive :omposition. These plates werethen further electrophotographically processed to the point of rinsingby electrortatically negatively charging the plates, followed byexposing them to a light imageand contacting the plates so- :xposed withan aluminum octanoate cross-linking promoting catalyst suspended in acarrier liquid.

A 10 liter capacity rinse tank of the type hereinbefore described wasthen readied by charging it to capacity with the present novel rinsecomposition comprising 99.0 parts by volume of equal volumes of FreonTF, the fiuorinated hydrocarbon, and the Isopar E, isoparaffinichydrocarbon, and 1 part of the odorless mineral spirits. The aboveprepared plates were then rinsed therein by immersing and withdrawingthem out through the spray curtain provided in the machine about 4times, with each immersion and withdrawal sequence being about 3 secondsin duration. After rinsing, each plate was allowed to completely dry inair for about minutes. Subsequent processing of the plates, includingpowderless etching, produced excellent quality image areas, that is, theareas covered by a photoresist and also, clear, substantiallyresistfree, non-image areas.

This example shows that a good rinsing action was obtained in thenon-image areas while yet not washing off detrimental quantities of thecatalyst deposition in the image areas. This together with its safetyfeatures, that is, its high flash point and relatively low toxicity,provides a new and novel rinse composition highly suitable for use inmaking high quality photoengraving plates.

Example II The procedure and activity of Example I Was repeated exceptthat the plates were immersed and withdrawn in and out of the rinseabout times. In this case the rinsing action was good in the non-imageareas but the images produced were of a slightly lesser quality thanpreviously obtained, but still usable, showing that more catalyst wasremoved from the coating in the image areas than in Example I.

Example III Example I was again repeated except that the plates wereimmersed and withdrawn twice. In this case, the images producedsubsequently were usable and of acceptable quality but were not sharplydefined since a less than desirable amount of catalyst was removed fromthe nonimage areas. Accordingly, the non-image areas after etchingexhibited more pimple-like projections than when like plates were rinsed4 times as in Example I.

The immersion times may vary from those used in the examples dependingon the number of rinses (immersionwithdrawals) employed, strength of therinse, and the like.

Example IV The procedure and activity of Example I was repeated exceptthat the rinse composition used comprised 47.5 parts by volume of boththe Freon TF, fluorinated hydrocarbon, and Isopar E, isoparaffinichydrocarbon, and 5 parts by volume of odorless mineral spirits. Whenused as a rinse as described resultant high quality photoengravingplates were produced having clean sharp images and clean non-image areasessentially free of the resist composition.

Example V Similarly as in Example IV a number of recording elements wererinsed in the present rinse composition but comprising 49.75 parts byvolume of both the Freon TF and Isopar E components and also 0.5 part byvolume of odorless mineral spirits. High quality resultantphotoengraving plates were again claimed.

It is manifest that various modifications can be made in the process ofthe present invention without departing from the spirit or scope thereofand it is understood that I limit myself only as defined in the appendedclaims.

I claim:

1. In the electrophotographic preparation of etchable photoengravingplates including; (a) providing a photoconductive compositioncomprising, a photo-conductive zinc oxide suspended in a cross-linkingresinous insulating binder coated on a metal plate, (b)electrostatically charging said coated plate, (c) exposing the chargedplate to an illuminated image to form a latent electrostatic image, (d)contacting said latent image with a cross-linking promoting catalystmaterial, (e) rinsing the plate socontacted in a rinsing liquid, (f)curing the plate at elevated temperatures, (g) removing the non-imageareas, and (h) contacting the so-treated plate with a powderless etchingbath; the improvement comprising, rinsing the catalyst-contacted platewith a rinse composition comprising: (1) from about 47.5 to about 49.75parts by volume of a stable, low toxicity, non-flammable fluorinatedhydrocarbon liquid characterized by a Kauri-Butanol value of about 31,an evaporation rate of 170 seconds (based on CCL, being a boiling pointof about 117.63 F. and a molecular weight of about 187.39; (2) fromabout 47.5 to about 49.75 parts by volume of a high purity, lowtoxicity, isoparafiinic solvent, having a Kauri-Butanol value of about29, an evaporation rate of about 353 seconds, and a flash point (TCC) ofabout 50 F.; and (3) from about 0.5 to about 5 parts by volume ofodorless mineral spirits characterized by a Kauri-Butanol value of about25, an evaporation rate of about 2400 seconds, and a flash point ofabout 133 F. (TCC).

2. The improvement of claim 1 wherein the rinsing step comprisescontacting said element with the rinsing composition by immersing andwithdrawing said plate into and out of said composition from about 2 toabout 10 times.

3. The improvement of claim 2 wherein the element is immersed andwithdrawn from about 3 to about 4 times.

4. The improvement of claim 2 wherein the period for each immersion andwithdrawal of the element for the rinse is from about 3 to about 8seconds.

5. A method of rinsing an electrostatically charged electrophotographicrecording element comprising a photoconductive zinc oxide dispersed in across-linking insulating resin as a photoconductive mixture coated on ametal plate, which element has first been exposed to an illuminatedimage followed by contact with a cross-linking promoting catalystcomprising; immersing and withdrawing said element through a liquidcurtain of a rinsing composition from about 2 to about 1-0 times, saidrinsing composition comprising: (1) from about 47.5 to about 49.75 partsby volume of a stable, low toxicity, non-flammable fiuorinatedhydrocarbon liquid characterized by a Kauri- Butanol value of about 31,an evaporation rate of (based on CCL, being 100), a boiling point ofabout 117.63 P. and a molecular weight of about 187.39; (2) from about47.5 to about 49.75 parts by volume of a high purity, low toxicity,isoparafiinic solvent, having a Kauri- Butanol value of about 29.0, anevaporation rate of about 353, and a flash point (TCC) of about 50 F.;and (3) from about 0.5 to about 5 parts by volume of odorless mineralspirits characterized by a Kauri-Butanol value of 5 about 25, anevaporation rate of about 2400, and a flash point of about 133 F. (TCC).

6. The method of claim 5 wherein the period for each immersion andwithdrawal of the element is from about 3 to about 8 seconds.

7. A rinse composition for use in the electro-photographic preparationof photoengraving plates comprising (a) from about 47.5 to about 49.75parts by volume of a non-flammable fluorinated hydrocarbon characterizedby a Kauri-Butanol value of about 31, an evaporation rate of about 170seconds, and a molecular weight of about 187.39, (b) from about 47.5 toabout 49.75 parts by volume of an isoparaflinic liquid characterized bya Kauri- Butanol value of about 29.0, an evaporation rate of about 353seconds, and a flash point of about 50 F., and (c) References CitedUNITED STATES PATENTS 3,062,684 11/1962 Nakamura 13440 X 3,215,52711/1965 Johnson 96--1 3,276,896 10/1966 Fisher 117-37 NORMAN G. TORCHIN,Primary Examiner. C. E. VAN HORN, Assistant Examiner.

1. IN THE ELECTROPHOTOGRAPHIC PREPARATION OF ETCHABLE PHOTOENGRAVINGPLATES INCLUDING: (A) PROVIDING A PHOTOCONDUCTIVE COMPOSITIONCOMPRISING, A PHOTOCONDUCTIVE ZINC OXIDE SUSPENDED IN A CROSS-LINKINGRESINOUS INSULATING BINDER COATED ON A METAL PLATE, (B)ELECTROSTATICALLY CHARGEDING SAID COATED PLATE, (C) EXPOSING THE CHARGEDPLATE TO AN ILLUMINATED IMAGE TO FORM A LATENT ELECTROSTIC IMAGE, (D)CONTACTING SAID LATENT IMAGE WITH A CROSS-LINKING PROMOTING CATALYSTMATERIAL, (E) RINSING THE PLATE SOCONTACTED IN A RINSING LIQUID, (F)CURING THE PLATE AT ELEVATED TEMPERATURES, (G) REMOVING THE NON-IMAGEAREAS, AND (H) CONTACTING THE SO-TREATED PLATE WITH A POWDERLESS ETCHINGBATH; THE IMPROVEMENT COMPRISING, RINSING THE CATALYST-CONTACTED PLATEWITH A RINSE COMPOSITION COMPRISING: (1) FROM ABOUT 47.5 TO ABOUT 49.75PARTS BY VOLUME OF A STABLE, LOW TOXICITY, NON-FLAMMABLE FLUORINATEDHYDROCARBON LIQUID CHARACTERIZED BY A KAURI-BUTANOL VALUE OF ABOUT 31,AN EVAPORATION RATE OF 170 SECONDS (BASED ON CCL4 BEING 100), A BOILINGPOINT OF ABOUT 117.63* F. AND A MOLECULAR WEIGHT OF ABOUT 187.39; (2)FROM ABOUT 47.5 TO ABOUT 49.75 PARTS BY VOLUME OF A HIGH PURITY, LOWTOXICITY, ISOPARAFFINIC SOLVENT, HAVING A KAURI-BUTANOL VALUE OF ABOUT29, AN EVAPORATION RATE OF ABOUT 353 SECONDS, AND A FLASH POINT (TCC) OFABOUT 50* F.; AND (3) FROM ABOUT 0.5 TO ABOUT 5 PARTS BY VOLUME OFODORLESS MINERAL SPIRITS CHARACTERIZED BY A KAURI-BUTANOL VALUE OF ABOUT25, AN EVAPORATION RATE OF ABOUT 2400 SECONDS, AND A FLASH POINT OFABOUT 133* F. (TCC).